What twist rate is good for me?
The longer the projectile, the faster the twist is needed in the barrel. Nowadays, the most common rate is 1x9 (one twist per nine inches), which is optimal for stabilizing 55 grain. 1x9 means one full revolution (twist) every 9 inches. This twist will adequately stabilize and 62-69 grain bullet but may have trouble with the heavier/longer 75 grain and up bullets. I cannot be exact here because every barrel manufacturer is different, and every barrel is different. This is just a general guideline.
The second most common twist is 1x7. Most milspec barrels are 1x7 because they need to stabilize the very long tracer rounds. It will adequately handle a 55 grain projectile. And handle 80 grain bullets and everything in between. The optimal projectile is about 75 grain. It should not be used for very short projectiles i.e. 40 and 46 grain projectiles.
The third most common twist is 1x8. This is usually used by precision oriented stainless barrels. The most notable exception is S&W’s MP15, which uses this rate. My personal belief is that this is a great middle of the road rate and should be used more often. It handles the 55 grain projectiles well as well as everything up to 77 grains. You would need to test it out for yourself. It’s optimal at about the 67 grain size.
There are slower twist barrels such as 1x12 twist. These barrels are optimal for smaller projectiles, like 36 and 40 grain bullets. These are usually for varmiters that use specialized bullets for their hunting needs.